diff --git a/.github/workflows/perf-nightly.yml b/.github/workflows/perf-nightly.yml
new file mode 100644
index 0000000000..3749df231a
--- /dev/null
+++ b/.github/workflows/perf-nightly.yml
@@ -0,0 +1,33 @@
+name: 'Performance Tests: Nightly'
+
+on:
+  schedule:
+    - cron: '0 3 * * *' # Runs at 3 AM every day
+  workflow_dispatch: # Allow manual trigger
+
+permissions:
+  contents: 'read'
+
+jobs:
+  perf-test:
+    name: 'Run Performance Usage Tests'
+    runs-on: 'gemini-cli-ubuntu-16-core'
+    if: "github.repository == 'google-gemini/gemini-cli'"
+    steps:
+      - name: 'Checkout'
+        uses: 'actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8' # ratchet:actions/checkout@v5
+
+      - name: 'Set up Node.js'
+        uses: 'actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020' # ratchet:actions/setup-node@v4
+        with:
+          node-version-file: '.nvmrc'
+          cache: 'npm'
+
+      - name: 'Install dependencies'
+        run: 'npm ci'
+
+      - name: 'Build project'
+        run: 'npm run build'
+
+      - name: 'Run Performance Tests'
+        run: 'npm run test:perf'
diff --git a/.gitignore b/.gitignore
index b6e3804ab5..85902b4a7c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -48,6 +48,7 @@ packages/cli/src/generated/
 packages/core/src/generated/
 packages/devtools/src/_client-assets.ts
 .integration-tests/
+.perf-tests/
 packages/vscode-ide-companion/*.vsix
 packages/cli/download-ripgrep*/
 
diff --git a/GEMINI.md b/GEMINI.md
index 60824972d3..4acdfc08be 100644
--- a/GEMINI.md
+++ b/GEMINI.md
@@ -44,8 +44,13 @@ powerful tool for developers.
 - **Test Commands:**
   - **Unit (All):** `npm run test`
   - **Integration (E2E):** `npm run test:e2e`
+  - > **NOTE**: Please run the memory and perf tests locally **only if** you are
+    > implementing changes related to those test areas. Otherwise skip these
+    > tests locally and rely on CI to run them on nightly builds.
   - **Memory (Nightly):** `npm run test:memory` (Runs memory regression tests
     against baselines. Excluded from `preflight`, run nightly.)
+  - **Performance (Nightly):** `npm run test:perf` (Runs CPU performance
+    regression tests against baselines. Excluded from `preflight`, run nightly.)
   - **Workspace-Specific:** `npm test -w <pkg> -- <path>` (Note: `<path>` must
     be relative to the workspace root, e.g.,
     `-w @google/gemini-cli-core -- src/routing/modelRouterService.test.ts`)
diff --git a/docs/integration-tests.md b/docs/integration-tests.md
index bfed813ebc..ddd4eb9c73 100644
--- a/docs/integration-tests.md
+++ b/docs/integration-tests.md
@@ -157,6 +157,48 @@ The harness (`MemoryTestHarness` in `packages/test-utils`):
 - Compares against baselines with a 10% tolerance.
 - Can analyze sustained leaks across 3 snapshots using `analyzeSnapshots()`.
 
+## Performance regression tests
+
+Performance regression tests are designed to detect wall-clock time, CPU usage,
+and event loop delay regressions across key CLI scenarios. They are located in
+the `perf-tests` directory.
+
+These tests are distinct from standard integration tests because they measure
+performance metrics and compare it against committed baselines.
+
+### Running performance tests
+
+Performance tests are not run as part of the default `npm run test` or
+`npm run test:e2e` commands. They are run nightly in CI but can be run manually:
+
+```bash
+npm run test:perf
+```
+
+### Updating baselines
+
+If you intentionally change behavior that affects performance, you may need to
+update the baselines. Set the `UPDATE_PERF_BASELINES` environment variable to
+`true`:
+
+```bash
+UPDATE_PERF_BASELINES=true npm run test:perf
+```
+
+This will run the tests multiple times (with warmup), apply IQR outlier
+filtering, and overwrite `perf-tests/baselines.json`. You should review the
+changes and commit the updated baseline file.
+
+### How it works
+
+The harness (`PerfTestHarness` in `packages/test-utils`):
+
+- Measures wall-clock time using `performance.now()`.
+- Measures CPU usage using `process.cpuUsage()`.
+- Monitors event loop delay using `perf_hooks.monitorEventLoopDelay()`.
+- Applies IQR (Interquartile Range) filtering to remove outlier samples.
+- Compares against baselines with a 15% tolerance.
+
 ## Diagnostics
 
 The integration test runner provides several options for diagnostics to help
diff --git a/integration-tests/globalSetup.ts b/integration-tests/globalSetup.ts
index 9dad51f9b3..4a15d03255 100644
--- a/integration-tests/globalSetup.ts
+++ b/integration-tests/globalSetup.ts
@@ -14,6 +14,7 @@ import { join, dirname, extname } from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { canUseRipgrep } from '../packages/core/src/tools/ripGrep.js';
 import { disableMouseTracking } from '@google/gemini-cli-core';
+import { isolateTestEnv } from '../packages/test-utils/src/env-setup.js';
 import { createServer, type Server } from 'node:http';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
@@ -88,15 +89,8 @@ export async function setup() {
   runDir = join(integrationTestsDir, `${Date.now()}`);
   await mkdir(runDir, { recursive: true });
 
-  // Set the home directory to the test run directory to avoid conflicts
-  // with the user's local config.
-  process.env['HOME'] = runDir;
-  if (process.platform === 'win32') {
-    process.env['USERPROFILE'] = runDir;
-  }
-  // We also need to set the config dir explicitly, since the code might
-  // construct the path before the HOME env var is set.
-  process.env['GEMINI_CONFIG_DIR'] = join(runDir, '.gemini');
+  // Isolate environment variables
+  isolateTestEnv(runDir);
 
   // Download ripgrep to avoid race conditions in parallel tests
   const available = await canUseRipgrep();
@@ -127,10 +121,6 @@ export async function setup() {
   }
 
   process.env['INTEGRATION_TEST_FILE_DIR'] = runDir;
-  process.env['GEMINI_CLI_INTEGRATION_TEST'] = 'true';
-  // Force file storage to avoid keychain prompts/hangs in CI, especially on macOS
-  process.env['GEMINI_FORCE_FILE_STORAGE'] = 'true';
-  process.env['TELEMETRY_LOG_FILE'] = join(runDir, 'telemetry.log');
 
   if (process.env['KEEP_OUTPUT']) {
     console.log(`Keeping output for test run in: ${runDir}`);
diff --git a/package-lock.json b/package-lock.json
index 2d3e670b74..3a22da1337 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -36,6 +36,7 @@
         "@types/ws": "^8.18.1",
         "@vitest/coverage-v8": "^3.1.1",
         "@vitest/eslint-plugin": "^1.3.4",
+        "asciichart": "^1.5.25",
         "cross-env": "^7.0.3",
         "depcheck": "^1.4.7",
         "domexception": "^4.0.0",
diff --git a/package.json b/package.json
index f531b41dbc..77801eaa7b 100644
--- a/package.json
+++ b/package.json
@@ -53,6 +53,8 @@
     "test:integration:sandbox:none": "cross-env GEMINI_SANDBOX=false vitest run --root ./integration-tests",
     "test:memory": "vitest run --root ./memory-tests",
     "test:memory:update-baselines": "cross-env UPDATE_MEMORY_BASELINES=true vitest run --root ./memory-tests",
+    "test:perf": "vitest run --root ./perf-tests",
+    "test:perf:update-baselines": "cross-env UPDATE_PERF_BASELINES=true vitest run --root ./perf-tests",
     "test:integration:sandbox:docker": "cross-env GEMINI_SANDBOX=docker npm run build:sandbox && cross-env GEMINI_SANDBOX=docker vitest run --root ./integration-tests",
     "test:integration:sandbox:podman": "cross-env GEMINI_SANDBOX=podman vitest run --root ./integration-tests",
     "lint": "cross-env NODE_OPTIONS=\"--max-old-space-size=8192\" eslint . --cache --max-warnings 0",
@@ -105,6 +107,7 @@
     "@types/ws": "^8.18.1",
     "@vitest/coverage-v8": "^3.1.1",
     "@vitest/eslint-plugin": "^1.3.4",
+    "asciichart": "^1.5.25",
     "cross-env": "^7.0.3",
     "depcheck": "^1.4.7",
     "domexception": "^4.0.0",
diff --git a/packages/test-utils/src/env-setup.ts b/packages/test-utils/src/env-setup.ts
new file mode 100644
index 0000000000..1c5ffd0d21
--- /dev/null
+++ b/packages/test-utils/src/env-setup.ts
@@ -0,0 +1,35 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { join } from 'node:path';
+
+/**
+ * Isolate the test environment by setting environment variables
+ * to point to a temporary run directory.
+ *
+ * @param runDir - The temporary directory for this test run.
+ */
+export function isolateTestEnv(runDir: string): void {
+  // Set the home directory to the test run directory to avoid conflicts
+  // with the user's local config.
+  process.env['HOME'] = runDir;
+  if (process.platform === 'win32') {
+    process.env['USERPROFILE'] = runDir;
+  }
+
+  // We also need to set the config dir explicitly, since the code might
+  // construct the path before the HOME env var is set.
+  process.env['GEMINI_CONFIG_DIR'] = join(runDir, '.gemini');
+
+  // Force file storage to avoid keychain prompts/hangs in CI, especially on macOS
+  process.env['GEMINI_FORCE_FILE_STORAGE'] = 'true';
+
+  // Mark as integration test
+  process.env['GEMINI_CLI_INTEGRATION_TEST'] = 'true';
+
+  // Isolate telemetry log
+  process.env['TELEMETRY_LOG_FILE'] = join(runDir, 'telemetry.log');
+}
diff --git a/packages/test-utils/src/index.ts b/packages/test-utils/src/index.ts
index 49eaec66d3..e851e7ab8d 100644
--- a/packages/test-utils/src/index.ts
+++ b/packages/test-utils/src/index.ts
@@ -8,6 +8,8 @@ export * from './file-system-test-helpers.js';
 export * from './fixtures/agents.js';
 export * from './memory-baselines.js';
 export * from './memory-test-harness.js';
+export * from './perf-test-harness.js';
 export * from './mock-utils.js';
 export * from './test-mcp-server.js';
 export * from './test-rig.js';
+export * from './env-setup.js';
diff --git a/packages/test-utils/src/perf-test-harness.ts b/packages/test-utils/src/perf-test-harness.ts
new file mode 100644
index 0000000000..c4625077be
--- /dev/null
+++ b/packages/test-utils/src/perf-test-harness.ts
@@ -0,0 +1,546 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { performance } from 'node:perf_hooks';
+import { setTimeout as sleep } from 'node:timers/promises';
+import { readFileSync, writeFileSync, existsSync } from 'node:fs';
+
+/** Configuration for asciichart plot function. */
+interface PlotConfig {
+  height?: number;
+  format?: (x: number) => string;
+}
+
+/** Type for the asciichart plot function. */
+type PlotFn = (series: number[], config?: PlotConfig) => string;
+
+/**
+ * Baseline entry for a single performance test scenario.
+ */
+export interface PerfBaseline {
+  wallClockMs: number;
+  cpuTotalUs: number;
+  eventLoopDelayP99Ms: number;
+  timestamp: string;
+}
+
+/**
+ * Top-level structure of the perf baselines JSON file.
+ */
+export interface PerfBaselineFile {
+  version: number;
+  updatedAt: string;
+  scenarios: Record<string, PerfBaseline>;
+}
+
+/**
+ * A single performance snapshot at a point in time.
+ */
+export interface PerfSnapshot {
+  timestamp: number;
+  label: string;
+  wallClockMs: number;
+  cpuUserUs: number;
+  cpuSystemUs: number;
+  cpuTotalUs: number;
+  eventLoopDelayP50Ms: number;
+  eventLoopDelayP95Ms: number;
+  eventLoopDelayP99Ms: number;
+  eventLoopDelayMaxMs: number;
+}
+
+/**
+ * Result from running a performance test scenario.
+ */
+export interface PerfTestResult {
+  scenarioName: string;
+  samples: PerfSnapshot[];
+  filteredSamples: PerfSnapshot[];
+  median: PerfSnapshot;
+  baseline: PerfBaseline | undefined;
+  withinTolerance: boolean;
+  deltaPercent: number;
+  cpuDeltaPercent: number;
+}
+
+/**
+ * Options for the PerfTestHarness.
+ */
+export interface PerfTestHarnessOptions {
+  /** Path to the baselines JSON file */
+  baselinesPath: string;
+  /** Default tolerance percentage (0-100). Default: 15 */
+  defaultTolerancePercent?: number;
+  /** Default CPU tolerance percentage (0-100). Optional */
+  defaultCpuTolerancePercent?: number;
+  /** Number of samples per scenario. Default: 5 */
+  sampleCount?: number;
+  /** Number of warmup runs to discard. Default: 1 */
+  warmupCount?: number;
+  /** Pause in ms between samples. Default: 100 */
+  samplePauseMs?: number;
+}
+
+/**
+ * Active timer state tracked internally.
+ */
+interface ActiveTimer {
+  label: string;
+  startTime: number;
+  startCpuUsage: NodeJS.CpuUsage;
+}
+
+/**
+ * PerfTestHarness provides infrastructure for running CPU performance tests.
+ *
+ * It handles:
+ * - High-resolution wall-clock timing via performance.now()
+ * - CPU usage measurement via process.cpuUsage()
+ * - Event loop delay monitoring via perf_hooks.monitorEventLoopDelay()
+ * - IQR outlier filtering for noise reduction
+ * - Warmup runs to avoid JIT compilation noise
+ * - Comparing against baselines with configurable tolerance
+ * - Generating ASCII chart reports
+ */
+export class PerfTestHarness {
+  private baselines: PerfBaselineFile;
+  private readonly baselinesPath: string;
+  private readonly defaultTolerancePercent: number;
+  private readonly defaultCpuTolerancePercent?: number;
+  private readonly sampleCount: number;
+  private readonly warmupCount: number;
+  private readonly samplePauseMs: number;
+  private allResults: PerfTestResult[] = [];
+  private activeTimers: Map<string, ActiveTimer> = new Map();
+
+  constructor(options: PerfTestHarnessOptions) {
+    this.baselinesPath = options.baselinesPath;
+    this.defaultTolerancePercent = options.defaultTolerancePercent ?? 15;
+    this.defaultCpuTolerancePercent = options.defaultCpuTolerancePercent;
+    this.sampleCount = options.sampleCount ?? 5;
+    this.warmupCount = options.warmupCount ?? 1;
+    this.samplePauseMs = options.samplePauseMs ?? 100;
+    this.baselines = loadPerfBaselines(this.baselinesPath);
+  }
+
+  /**
+   * Start a high-resolution timer with CPU tracking.
+   */
+  startTimer(label: string): void {
+    this.activeTimers.set(label, {
+      label,
+      startTime: performance.now(),
+      startCpuUsage: process.cpuUsage(),
+    });
+  }
+
+  /**
+   * Stop a timer and return the snapshot.
+   */
+  stopTimer(label: string): PerfSnapshot {
+    const timer = this.activeTimers.get(label);
+    if (!timer) {
+      throw new Error(`No active timer found for label "${label}"`);
+    }
+
+    const wallClockMs = performance.now() - timer.startTime;
+    const cpuDelta = process.cpuUsage(timer.startCpuUsage);
+    this.activeTimers.delete(label);
+
+    return {
+      timestamp: Date.now(),
+      label,
+      wallClockMs,
+      cpuUserUs: cpuDelta.user,
+      cpuSystemUs: cpuDelta.system,
+      cpuTotalUs: cpuDelta.user + cpuDelta.system,
+      eventLoopDelayP50Ms: 0,
+      eventLoopDelayP95Ms: 0,
+      eventLoopDelayP99Ms: 0,
+      eventLoopDelayMaxMs: 0,
+    };
+  }
+
+  /**
+   * Measure a function's wall-clock time and CPU usage.
+   * Returns the snapshot with timing data.
+   */
+  async measure(label: string, fn: () => Promise<void>): Promise<PerfSnapshot> {
+    this.startTimer(label);
+    await fn();
+    return this.stopTimer(label);
+  }
+
+  /**
+   * Measure a function with event loop delay monitoring.
+   * Uses perf_hooks.monitorEventLoopDelay() for histogram data.
+   */
+  async measureWithEventLoop(
+    label: string,
+    fn: () => Promise<void>,
+  ): Promise<PerfSnapshot> {
+    // monitorEventLoopDelay is available in Node.js 12+
+    const { monitorEventLoopDelay } = await import('node:perf_hooks');
+    const histogram = monitorEventLoopDelay({ resolution: 10 });
+    histogram.enable();
+
+    this.startTimer(label);
+    await fn();
+    const snapshot = this.stopTimer(label);
+
+    histogram.disable();
+
+    // Convert from nanoseconds to milliseconds
+    snapshot.eventLoopDelayP50Ms = histogram.percentile(50) / 1e6;
+    snapshot.eventLoopDelayP95Ms = histogram.percentile(95) / 1e6;
+    snapshot.eventLoopDelayP99Ms = histogram.percentile(99) / 1e6;
+    snapshot.eventLoopDelayMaxMs = histogram.max / 1e6;
+
+    return snapshot;
+  }
+
+  /**
+   * Run a scenario multiple times with warmup, outlier filtering, and baseline comparison.
+   *
+   * @param name - Scenario name (must match baseline key)
+   * @param fn - Async function that executes one sample of the scenario.
+   *             Must return a PerfSnapshot with measured values.
+   * @param tolerancePercent - Override default tolerance for this scenario
+   */
+  async runScenario(
+    name: string,
+    fn: () => Promise<PerfSnapshot>,
+    tolerancePercent?: number,
+  ): Promise<PerfTestResult> {
+    const tolerance = tolerancePercent ?? this.defaultTolerancePercent;
+    const totalRuns = this.warmupCount + this.sampleCount;
+    const allSnapshots: PerfSnapshot[] = [];
+
+    for (let i = 0; i < totalRuns; i++) {
+      const isWarmup = i < this.warmupCount;
+      const snapshot = await fn();
+      snapshot.label = isWarmup
+        ? `warmup-${i}`
+        : `sample-${i - this.warmupCount}`;
+
+      if (!isWarmup) {
+        allSnapshots.push(snapshot);
+      }
+
+      // Brief pause between samples
+      await sleep(this.samplePauseMs);
+    }
+
+    // Apply IQR outlier filtering on wall-clock time
+    const filteredSnapshots = this.filterOutliers(allSnapshots, 'wallClockMs');
+
+    // Get median of filtered samples
+    const median = this.getMedianSnapshot(filteredSnapshots);
+    median.label = 'median';
+
+    // Get baseline
+    const baseline = this.baselines.scenarios[name];
+
+    // Determine if within tolerance
+    let deltaPercent = 0;
+    let cpuDeltaPercent = 0;
+    let withinTolerance = true;
+
+    if (baseline) {
+      deltaPercent =
+        ((median.wallClockMs - baseline.wallClockMs) / baseline.wallClockMs) *
+        100;
+      cpuDeltaPercent =
+        ((median.cpuTotalUs - baseline.cpuTotalUs) / baseline.cpuTotalUs) * 100;
+      withinTolerance = deltaPercent <= tolerance;
+    }
+
+    const result: PerfTestResult = {
+      scenarioName: name,
+      samples: allSnapshots,
+      filteredSamples: filteredSnapshots,
+      median,
+      baseline,
+      withinTolerance,
+      deltaPercent,
+      cpuDeltaPercent,
+    };
+
+    this.allResults.push(result);
+    return result;
+  }
+
+  /**
+   * Assert that a scenario result is within the baseline tolerance.
+   */
+  assertWithinBaseline(
+    result: PerfTestResult,
+    tolerancePercent?: number,
+    cpuTolerancePercent?: number,
+  ): void {
+    const tolerance = tolerancePercent ?? this.defaultTolerancePercent;
+    const cpuTolerance = cpuTolerancePercent ?? this.defaultCpuTolerancePercent;
+
+    if (!result.baseline) {
+      console.warn(
+        `⚠ No baseline found for "${result.scenarioName}". ` +
+          `Run with UPDATE_PERF_BASELINES=true to create one. ` +
+          `Measured: ${result.median.wallClockMs.toFixed(1)} ms wall-clock.`,
+      );
+      return;
+    }
+
+    const deltaPercent =
+      ((result.median.wallClockMs - result.baseline.wallClockMs) /
+        result.baseline.wallClockMs) *
+      100;
+
+    if (deltaPercent > tolerance) {
+      throw new Error(
+        `Performance regression detected for "${result.scenarioName}"!\n` +
+          `  Measured:    ${result.median.wallClockMs.toFixed(1)} ms wall-clock\n` +
+          `  Baseline:    ${result.baseline.wallClockMs.toFixed(1)} ms wall-clock\n` +
+          `  Delta:       ${deltaPercent.toFixed(1)}% (tolerance: ${tolerance}%)\n` +
+          `  CPU total:   ${formatUs(result.median.cpuTotalUs)}\n` +
+          `  EL p99:      ${result.median.eventLoopDelayP99Ms.toFixed(1)} ms\n` +
+          `  Samples:     ${result.samples.length} (${result.filteredSamples.length} after IQR filter)`,
+      );
+    }
+
+    if (cpuTolerance !== undefined && result.cpuDeltaPercent > cpuTolerance) {
+      throw new Error(
+        `CPU usage regression detected for "${result.scenarioName}"!\n` +
+          `  Measured:    ${formatUs(result.median.cpuTotalUs)}\n` +
+          `  Baseline:    ${formatUs(result.baseline.cpuTotalUs)}\n` +
+          `  Delta:       ${result.cpuDeltaPercent.toFixed(1)}% (tolerance: ${cpuTolerance}%)\n` +
+          `  Wall-clock:  ${result.median.wallClockMs.toFixed(1)} ms\n` +
+          `  EL p99:      ${result.median.eventLoopDelayP99Ms.toFixed(1)} ms`,
+      );
+    }
+  }
+
+  /**
+   * Update the baseline for a scenario with the current measured values.
+   */
+  updateScenarioBaseline(result: PerfTestResult): void {
+    updatePerfBaseline(this.baselinesPath, result.scenarioName, {
+      wallClockMs: result.median.wallClockMs,
+      cpuTotalUs: result.median.cpuTotalUs,
+      eventLoopDelayP99Ms: result.median.eventLoopDelayP99Ms,
+    });
+    // Reload baselines after update
+    this.baselines = loadPerfBaselines(this.baselinesPath);
+    console.log(
+      `Updated baseline for ${result.scenarioName}: ${result.median.wallClockMs.toFixed(1)} ms`,
+    );
+  }
+
+  /**
+   * Generate an ASCII report with summary table and charts.
+   */
+  async generateReport(results?: PerfTestResult[]): Promise<string> {
+    const resultsToReport = results ?? this.allResults;
+    const lines: string[] = [];
+
+    lines.push('');
+    lines.push('═══════════════════════════════════════════════════');
+    lines.push('         PERFORMANCE TEST REPORT');
+    lines.push('═══════════════════════════════════════════════════');
+    lines.push('');
+
+    for (const result of resultsToReport) {
+      const measured = `${result.median.wallClockMs.toFixed(1)} ms`;
+      const baseline = result.baseline
+        ? `${result.baseline.wallClockMs.toFixed(1)} ms`
+        : 'N/A';
+      const delta = result.baseline
+        ? `${result.deltaPercent >= 0 ? '+' : ''}${result.deltaPercent.toFixed(1)}%`
+        : 'N/A';
+      const status = !result.baseline
+        ? 'NEW'
+        : result.withinTolerance
+          ? '✅'
+          : '❌';
+
+      lines.push(
+        `${result.scenarioName}: ${measured} (Baseline: ${baseline}, Delta: ${delta}) ${status}`,
+      );
+
+      // Show CPU breakdown
+      const cpuMs = `${(result.median.cpuTotalUs / 1000).toFixed(1)} ms`;
+      lines.push(
+        `  CPU: ${cpuMs} (user: ${formatUs(result.median.cpuUserUs)}, system: ${formatUs(result.median.cpuSystemUs)})`,
+      );
+
+      if (result.median.eventLoopDelayP99Ms > 0) {
+        lines.push(
+          `  Event loop: p50=${result.median.eventLoopDelayP50Ms.toFixed(1)}ms p95=${result.median.eventLoopDelayP95Ms.toFixed(1)}ms p99=${result.median.eventLoopDelayP99Ms.toFixed(1)}ms max=${result.median.eventLoopDelayMaxMs.toFixed(1)}ms`,
+        );
+      }
+
+      lines.push(
+        `  Samples: ${result.samples.length} → ${result.filteredSamples.length} after IQR filter`,
+      );
+    }
+    lines.push('');
+
+    // Generate ASCII chart for wall-clock per scenario
+    try {
+      // @ts-expect-error - asciichart may not have types
+      const asciichart = (await import('asciichart')) as {
+        default?: { plot?: PlotFn };
+        plot?: PlotFn;
+      };
+      const plot: PlotFn | undefined =
+        asciichart.default?.plot ?? asciichart.plot;
+
+      for (const result of resultsToReport) {
+        if (result.filteredSamples.length > 2) {
+          lines.push(`📈 Wall-clock trend: ${result.scenarioName}`);
+          lines.push('─'.repeat(60));
+
+          const wallClockData = result.filteredSamples.map(
+            (s) => s.wallClockMs,
+          );
+
+          if (plot) {
+            const chart = plot(wallClockData, {
+              height: 8,
+              format: (x: number) => `${x.toFixed(0)} ms`.padStart(10),
+            });
+            lines.push(chart);
+          }
+
+          const labels = result.filteredSamples.map((s) => s.label);
+          lines.push('  ' + labels.join(' → '));
+          lines.push('');
+        }
+      }
+    } catch {
+      lines.push(
+        '(asciichart not available — install with: npm install --save-dev asciichart)',
+      );
+      lines.push('');
+    }
+
+    lines.push('═══════════════════════════════════════════════════');
+    lines.push('');
+
+    const report = lines.join('\n');
+    console.log(report);
+    return report;
+  }
+
+  /**
+   * Filter outliers using the Interquartile Range (IQR) method.
+   * Removes samples where the given metric falls outside Q1 - 1.5*IQR or Q3 + 1.5*IQR.
+   */
+  private filterOutliers(
+    snapshots: PerfSnapshot[],
+    metric: keyof PerfSnapshot,
+  ): PerfSnapshot[] {
+    if (snapshots.length < 4) {
+      // Not enough data for meaningful IQR filtering
+      return [...snapshots];
+    }
+
+    const sorted = [...snapshots].sort(
+      (a, b) => (a[metric] as number) - (b[metric] as number),
+    );
+    const q1Idx = Math.floor(sorted.length * 0.25);
+    const q3Idx = Math.floor(sorted.length * 0.75);
+
+    const q1 = sorted[q1Idx]![metric] as number;
+    const q3 = sorted[q3Idx]![metric] as number;
+    const iqr = q3 - q1;
+    const lowerBound = q1 - 1.5 * iqr;
+    const upperBound = q3 + 1.5 * iqr;
+
+    return snapshots.filter((s) => {
+      const val = s[metric] as number;
+      return val >= lowerBound && val <= upperBound;
+    });
+  }
+
+  /**
+   * Get the median snapshot by wall-clock time from a sorted list.
+   */
+  private getMedianSnapshot(snapshots: PerfSnapshot[]): PerfSnapshot {
+    if (snapshots.length === 0) {
+      throw new Error('Cannot compute median of empty snapshot list');
+    }
+
+    const sorted = [...snapshots].sort((a, b) => a.wallClockMs - b.wallClockMs);
+    const medianIdx = Math.floor(sorted.length / 2);
+    return { ...sorted[medianIdx]! };
+  }
+}
+
+// ─── Baseline management ─────────────────────────────────────────────
+
+/**
+ * Load perf baselines from a JSON file.
+ */
+export function loadPerfBaselines(path: string): PerfBaselineFile {
+  if (!existsSync(path)) {
+    return {
+      version: 1,
+      updatedAt: new Date().toISOString(),
+      scenarios: {},
+    };
+  }
+
+  const content = readFileSync(path, 'utf-8');
+  return JSON.parse(content) as PerfBaselineFile;
+}
+
+/**
+ * Save perf baselines to a JSON file.
+ */
+export function savePerfBaselines(
+  path: string,
+  baselines: PerfBaselineFile,
+): void {
+  baselines.updatedAt = new Date().toISOString();
+  writeFileSync(path, JSON.stringify(baselines, null, 2) + '\n');
+}
+
+/**
+ * Update (or create) a single scenario baseline in the file.
+ */
+export function updatePerfBaseline(
+  path: string,
+  scenarioName: string,
+  measured: {
+    wallClockMs: number;
+    cpuTotalUs: number;
+    eventLoopDelayP99Ms: number;
+  },
+): void {
+  const baselines = loadPerfBaselines(path);
+  baselines.scenarios[scenarioName] = {
+    wallClockMs: measured.wallClockMs,
+    cpuTotalUs: measured.cpuTotalUs,
+    eventLoopDelayP99Ms: measured.eventLoopDelayP99Ms,
+    timestamp: new Date().toISOString(),
+  };
+  savePerfBaselines(path, baselines);
+}
+
+// ─── Helpers ─────────────────────────────────────────────────────────
+
+/**
+ * Format microseconds as a human-readable string.
+ */
+function formatUs(us: number): string {
+  if (us > 1_000_000) {
+    return `${(us / 1_000_000).toFixed(2)} s`;
+  }
+  if (us > 1_000) {
+    return `${(us / 1_000).toFixed(1)} ms`;
+  }
+  return `${us} μs`;
+}
diff --git a/perf-tests/README.md b/perf-tests/README.md
new file mode 100644
index 0000000000..c8e9e448c1
--- /dev/null
+++ b/perf-tests/README.md
@@ -0,0 +1,121 @@
+# CPU Performance Integration Test Harness
+
+## Overview
+
+This directory contains performance/CPU integration tests for the Gemini CLI.
+These tests measure wall-clock time, CPU usage, and event loop responsiveness to
+detect regressions across key scenarios.
+
+CPU performance is inherently noisy, especially in CI. The harness addresses
+this with:
+
+- **IQR outlier filtering** — discards anomalous samples
+- **Median sampling** — takes N runs, reports the median after filtering
+- **Warmup runs** — discards the first run to mitigate JIT compilation noise
+- **15% default tolerance** — won't panic at slight regressions
+
+## Running
+
+```bash
+# Run tests (compare against committed baselines)
+npm run test:perf
+
+# Update baselines (after intentional changes)
+npm run test:perf:update-baselines
+
+# Verbose output
+VERBOSE=true npm run test:perf
+
+# Keep test artifacts for debugging
+KEEP_OUTPUT=true npm run test:perf
+```
+
+## How It Works
+
+### Measurement Primitives
+
+The `PerfTestHarness` class (in `packages/test-utils`) provides:
+
+- **`performance.now()`** — high-resolution wall-clock timing
+- **`process.cpuUsage()`** — user + system CPU microseconds (delta between
+  start/stop)
+- **`perf_hooks.monitorEventLoopDelay()`** — event loop delay histogram
+  (p50/p95/p99/max)
+
+### Noise Reduction
+
+1. **Warmup**: First run is discarded to mitigate JIT compilation artifacts
+2. **Multiple samples**: Each scenario runs N times (default 5)
+3. **IQR filtering**: Samples outside Q1−1.5×IQR and Q3+1.5×IQR are discarded
+4. **Median**: The median of remaining samples is used for comparison
+
+### Baseline Management
+
+Baselines are stored in `baselines.json` in this directory. Each scenario has:
+
+```json
+{
+  "cold-startup-time": {
+    "wallClockMs": 1234.5,
+    "cpuTotalUs": 567890,
+    "eventLoopDelayP99Ms": 12.3,
+    "timestamp": "2026-04-08T..."
+  }
+}
+```
+
+Tests fail if the measured value exceeds `baseline × 1.15` (15% tolerance).
+
+To recalibrate after intentional changes:
+
+```bash
+npm run test:perf:update-baselines
+# then commit baselines.json
+```
+
+### Report Output
+
+After all tests, the harness prints an ASCII summary:
+
+```
+═══════════════════════════════════════════════════
+         PERFORMANCE TEST REPORT
+═══════════════════════════════════════════════════
+
+cold-startup-time:   1234.5 ms (Baseline: 1200.0 ms, Delta: +2.9%) ✅
+idle-cpu-usage:         2.1 %  (Baseline: 2.0 %, Delta: +5.0%)     ✅
+skill-loading-time:  1567.8 ms (Baseline: 1500.0 ms, Delta: +4.5%) ✅
+```
+
+## Architecture
+
+```
+perf-tests/
+├── README.md              ← you are here
+├── baselines.json         ← committed baseline values
+├── globalSetup.ts         ← test environment setup
+├── perf-usage.test.ts     ← test scenarios
+├── perf.*.responses       ← fake API responses per scenario
+├── tsconfig.json          ← TypeScript config
+└── vitest.config.ts       ← vitest config (serial, isolated)
+
+packages/test-utils/src/
+├── perf-test-harness.ts   ← PerfTestHarness class
+└── index.ts               ← re-exports
+```
+
+## CI Integration
+
+These tests are **excluded from `preflight`** and designed for nightly CI:
+
+```yaml
+- name: Performance regression tests
+  run: npm run test:perf
+```
+
+## Adding a New Scenario
+
+1. Add a fake response file: `perf.<scenario-name>.responses`
+2. Add a test case in `perf-usage.test.ts` using `harness.runScenario()`
+3. Run `npm run test:perf:update-baselines` to establish initial baseline
+4. Commit the updated `baselines.json`
diff --git a/perf-tests/baselines.json b/perf-tests/baselines.json
new file mode 100644
index 0000000000..a6bad73574
--- /dev/null
+++ b/perf-tests/baselines.json
@@ -0,0 +1,24 @@
+{
+  "version": 1,
+  "updatedAt": "2026-04-08T18:51:29.839Z",
+  "scenarios": {
+    "cold-startup-time": {
+      "wallClockMs": 1333.4230420000004,
+      "cpuTotalUs": 1711,
+      "eventLoopDelayP99Ms": 0,
+      "timestamp": "2026-04-08T18:50:58.124Z"
+    },
+    "idle-cpu-usage": {
+      "wallClockMs": 5001.926125,
+      "cpuTotalUs": 128518,
+      "eventLoopDelayP99Ms": 12.705791,
+      "timestamp": "2026-04-08T18:51:23.938Z"
+    },
+    "skill-loading-time": {
+      "wallClockMs": 1372.4463749999995,
+      "cpuTotalUs": 1550,
+      "eventLoopDelayP99Ms": 0,
+      "timestamp": "2026-04-08T18:51:29.839Z"
+    }
+  }
+}
diff --git a/perf-tests/globalSetup.ts b/perf-tests/globalSetup.ts
new file mode 100644
index 0000000000..77447bd2ba
--- /dev/null
+++ b/perf-tests/globalSetup.ts
@@ -0,0 +1,67 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { mkdir, readdir, rm } from 'node:fs/promises';
+import { join, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { canUseRipgrep } from '../packages/core/src/tools/ripGrep.js';
+import { isolateTestEnv } from '../packages/test-utils/src/env-setup.js';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const rootDir = join(__dirname, '..');
+const perfTestsDir = join(rootDir, '.perf-tests');
+const KEEP_RUNS_COUNT = 5;
+let runDir = '';
+
+export async function setup() {
+  runDir = join(perfTestsDir, `${Date.now()}`);
+  await mkdir(runDir, { recursive: true });
+
+  // Isolate environment variables
+  isolateTestEnv(runDir);
+
+  // Download ripgrep to avoid race conditions
+  const available = await canUseRipgrep();
+  if (!available) {
+    throw new Error('Failed to download ripgrep binary');
+  }
+
+  // Clean up old test runs, keeping the latest few for debugging
+  try {
+    const testRuns = await readdir(perfTestsDir);
+    if (testRuns.length > KEEP_RUNS_COUNT) {
+      const oldRuns = testRuns
+        .sort()
+        .slice(0, testRuns.length - KEEP_RUNS_COUNT);
+      await Promise.all(
+        oldRuns.map((oldRun) =>
+          rm(join(perfTestsDir, oldRun), {
+            recursive: true,
+            force: true,
+          }),
+        ),
+      );
+    }
+  } catch (e) {
+    console.error('Error cleaning up old perf test runs:', e);
+  }
+
+  process.env['INTEGRATION_TEST_FILE_DIR'] = runDir;
+  process.env['VERBOSE'] = process.env['VERBOSE'] ?? 'false';
+
+  console.log(`\nPerf test output directory: ${runDir}`);
+}
+
+export async function teardown() {
+  // Cleanup unless KEEP_OUTPUT is set
+  if (process.env['KEEP_OUTPUT'] !== 'true' && runDir) {
+    try {
+      await rm(runDir, { recursive: true, force: true });
+    } catch (e) {
+      console.warn('Failed to clean up perf test directory:', e);
+    }
+  }
+}
diff --git a/perf-tests/perf-usage.test.ts b/perf-tests/perf-usage.test.ts
new file mode 100644
index 0000000000..3f92cd9f91
--- /dev/null
+++ b/perf-tests/perf-usage.test.ts
@@ -0,0 +1,153 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, beforeAll, afterAll } from 'vitest';
+import { TestRig, PerfTestHarness } from '@google/gemini-cli-test-utils';
+import { join, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const BASELINES_PATH = join(__dirname, 'baselines.json');
+const UPDATE_BASELINES = process.env['UPDATE_PERF_BASELINES'] === 'true';
+const TOLERANCE_PERCENT = 15;
+
+// Use fewer samples locally for faster iteration, more in CI
+const SAMPLE_COUNT = process.env['CI'] ? 5 : 3;
+const WARMUP_COUNT = 1;
+
+describe('CPU Performance Tests', () => {
+  let harness: PerfTestHarness;
+
+  beforeAll(() => {
+    harness = new PerfTestHarness({
+      baselinesPath: BASELINES_PATH,
+      defaultTolerancePercent: TOLERANCE_PERCENT,
+      sampleCount: SAMPLE_COUNT,
+      warmupCount: WARMUP_COUNT,
+    });
+  });
+
+  afterAll(async () => {
+    // Generate the summary report after all tests
+    await harness.generateReport();
+  });
+
+  it('cold-startup-time: startup completes within baseline', async () => {
+    const result = await harness.runScenario('cold-startup-time', async () => {
+      const rig = new TestRig();
+      try {
+        rig.setup('perf-cold-startup', {
+          fakeResponsesPath: join(__dirname, 'perf.cold-startup.responses'),
+        });
+
+        return await harness.measure('cold-startup', async () => {
+          await rig.run({
+            args: ['hello'],
+            timeout: 120000,
+            env: { GEMINI_API_KEY: 'fake-perf-test-key' },
+          });
+        });
+      } finally {
+        await rig.cleanup();
+      }
+    });
+
+    if (UPDATE_BASELINES) {
+      harness.updateScenarioBaseline(result);
+    } else {
+      harness.assertWithinBaseline(result);
+    }
+  });
+
+  it('idle-cpu-usage: CPU stays low when idle', async () => {
+    const IDLE_OBSERVATION_MS = 5000;
+
+    const result = await harness.runScenario('idle-cpu-usage', async () => {
+      const rig = new TestRig();
+      try {
+        rig.setup('perf-idle-cpu', {
+          fakeResponsesPath: join(__dirname, 'perf.idle-cpu.responses'),
+        });
+
+        // First, run a prompt to get the CLI into idle state
+        await rig.run({
+          args: ['hello'],
+          timeout: 120000,
+          env: { GEMINI_API_KEY: 'fake-perf-test-key' },
+        });
+
+        // Now measure CPU during idle period in the test process
+        return await harness.measureWithEventLoop('idle-cpu', async () => {
+          // Simulate idle period — just wait
+          const { setTimeout: sleep } = await import('node:timers/promises');
+          await sleep(IDLE_OBSERVATION_MS);
+        });
+      } finally {
+        await rig.cleanup();
+      }
+    });
+
+    if (UPDATE_BASELINES) {
+      harness.updateScenarioBaseline(result);
+    } else {
+      harness.assertWithinBaseline(result);
+    }
+  });
+
+  it('skill-loading-time: startup with many skills within baseline', async () => {
+    const SKILL_COUNT = 20;
+
+    const result = await harness.runScenario('skill-loading-time', async () => {
+      const rig = new TestRig();
+      try {
+        rig.setup('perf-skill-loading', {
+          fakeResponsesPath: join(__dirname, 'perf.skill-loading.responses'),
+        });
+
+        // Create many skill directories with SKILL.md files
+        for (let i = 0; i < SKILL_COUNT; i++) {
+          const skillDir = `.gemini/skills/perf-skill-${i}`;
+          rig.mkdir(skillDir);
+          rig.createFile(
+            `${skillDir}/SKILL.md`,
+            [
+              '---',
+              `name: perf-skill-${i}`,
+              `description: Performance test skill number ${i}`,
+              `activation: manual`,
+              '---',
+              '',
+              `# Performance Test Skill ${i}`,
+              '',
+              `This is a test skill for measuring skill loading performance.`,
+              `It contains some content to simulate real-world skill files.`,
+              '',
+              `## Usage`,
+              '',
+              `Use this skill by activating it with @perf-skill-${i}.`,
+            ].join('\n'),
+          );
+        }
+
+        return await harness.measure('skill-loading', async () => {
+          await rig.run({
+            args: ['hello'],
+            timeout: 120000,
+            env: { GEMINI_API_KEY: 'fake-perf-test-key' },
+          });
+        });
+      } finally {
+        await rig.cleanup();
+      }
+    });
+
+    if (UPDATE_BASELINES) {
+      harness.updateScenarioBaseline(result);
+    } else {
+      harness.assertWithinBaseline(result);
+    }
+  });
+});
diff --git a/perf-tests/perf.cold-startup.responses b/perf-tests/perf.cold-startup.responses
new file mode 100644
index 0000000000..7a5703e3d2
--- /dev/null
+++ b/perf-tests/perf.cold-startup.responses
@@ -0,0 +1,2 @@
+{"method":"generateContent","response":{"candidates":[{"content":{"parts":[{"text":"0"}],"role":"model"},"finishReason":"STOP","index":0}]}}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"Hello! I'm ready to help. What would you like to work on?"}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":5,"candidatesTokenCount":12,"totalTokenCount":17,"promptTokensDetails":[{"modality":"TEXT","tokenCount":5}]}}]}
diff --git a/perf-tests/perf.idle-cpu.responses b/perf-tests/perf.idle-cpu.responses
new file mode 100644
index 0000000000..a0d05086d2
--- /dev/null
+++ b/perf-tests/perf.idle-cpu.responses
@@ -0,0 +1,2 @@
+{"method":"generateContent","response":{"candidates":[{"content":{"parts":[{"text":"0"}],"role":"model"},"finishReason":"STOP","index":0}]}}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"Hello! I'm ready to help."}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":5,"candidatesTokenCount":8,"totalTokenCount":13,"promptTokensDetails":[{"modality":"TEXT","tokenCount":5}]}}]}
diff --git a/perf-tests/perf.skill-loading.responses b/perf-tests/perf.skill-loading.responses
new file mode 100644
index 0000000000..eb6c96fe9c
--- /dev/null
+++ b/perf-tests/perf.skill-loading.responses
@@ -0,0 +1,2 @@
+{"method":"generateContent","response":{"candidates":[{"content":{"parts":[{"text":"0"}],"role":"model"},"finishReason":"STOP","index":0}]}}
+{"method":"generateContentStream","response":[{"candidates":[{"content":{"parts":[{"text":"Hello! I'm ready to assist you with your project."}],"role":"model"},"finishReason":"STOP","index":0}],"usageMetadata":{"promptTokenCount":5,"candidatesTokenCount":10,"totalTokenCount":15,"promptTokensDetails":[{"modality":"TEXT","tokenCount":5}]}}]}
diff --git a/perf-tests/tsconfig.json b/perf-tests/tsconfig.json
new file mode 100644
index 0000000000..7f2c199703
--- /dev/null
+++ b/perf-tests/tsconfig.json
@@ -0,0 +1,12 @@
+{
+  "extends": "../tsconfig.json",
+  "compilerOptions": {
+    "noEmit": true,
+    "allowJs": true
+  },
+  "include": ["**/*.ts"],
+  "references": [
+    { "path": "../packages/core" },
+    { "path": "../packages/test-utils" }
+  ]
+}
diff --git a/perf-tests/vitest.config.ts b/perf-tests/vitest.config.ts
new file mode 100644
index 0000000000..e9baeec0bf
--- /dev/null
+++ b/perf-tests/vitest.config.ts
@@ -0,0 +1,27 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    testTimeout: 600000, // 10 minutes — performance profiling needs time for multiple samples
+    globalSetup: './globalSetup.ts',
+    reporters: ['default'],
+    include: ['**/*.test.ts'],
+    retry: 0, // No retries — noise is handled by IQR filtering and tolerance
+    fileParallelism: false, // Must run serially to avoid CPU contention
+    pool: 'forks',
+    poolOptions: {
+      forks: {
+        singleFork: true, // Single process for accurate per-test CPU readings
+      },
+    },
+    env: {
+      GEMINI_TEST_TYPE: 'perf',
+    },
+  },
+});